Vadose Zone Monitoring with a Stevens Hydra Probe II in a Typic Fragixeralf Sandy Clay Loam Soil.

The high costs of landslides and inefficient agricultural land use have increased demand on precision technology to accurately determine field conditions. Full characterization of the vadose zone, through simultaneous and continuous monitoring of soil moisture, temperature and electric conductivity provide important information necessary for environmental assessment, remediation and successful land management. In October 2006, Hydra Probes^Ò were installed in clay loam soil classified as a Typic Fragixeralf on a 25% forested slope in urban Portland, Oregon, USA.  Five horizons namely the A1, A2, Btx1, Bt2, Btx3 and Bgx were sampled to a depth of 110cm in situ and monitored using the Hydra Probes.  Porosity, field capacity and wilting point were calculated for each horizon and there was good agreement with field measured values. An estimate of the hydraulic conductivity of the horizons was determined through movement of the wetting front and monitoring of end of summer rainfall events under differing rainfall intensities showed vertical and lateral vadose zone percolation. Daily, monthly and annual temperature variability was clearly observed in each horizon, dampening with depth. The lowest fragipan (Bgx) had a higher temperature than 4 of the shallower horizons for about 6 months and soil surface temperature tracked atmospheric temperature with a lag time of two days. As much as a 10 degree difference was observed between the A1 and the Bgx horizons. There was an increase in conductivity with depth although it never exceeded 0.1 S/m for any of the horizons and high correlation with moisture content showed pore matrix effects. Several field observations of the Btx1 were consistent with it being the densest horizon.